1. Field of the Invention
The present invention relates to use of sea lettuce, a kind of seaweed, and its extract as feed and medicines. In particular, the invention relates to use of the antiviral component contained in the sea lettuce.
2. Description of the Related Art
In fish-farming, there has often seen the overcrowded cultivation that attached great importance to productive efficiency, and because of such the cultivation, various fish diseases to induce death of the cultivated fish have been reported. Among such fish diseases, various medicines such as antibiotics can control fish diseases in the event of onset caused by bacteria and so on, and the control of fish diseases by use of antibiotics has already matured into a particular industrial field in cultivation business.
On viral diseases, damages from various kinds of viral fish diseases has begun to be reported since 1980's. For example, flounder rhabdovirus (HRV: 200 nm overall length, bullet form with an envelope) spread over flounder farms in Hokkaido in the first half of 1980's, and it is reported that 80% of flounder fry died then. In addition, it is said that the amount of damage of salmon family fish by infectious hematopoietic necrosis virus (IHNV: 200 nm overall length, bullet form with an envelope) is estimated to be 2 to 3 billion yen/year, and several 10s billion yen/year in the world. So, many people who involve fish-farming are cautious universally thereon.
However, concerning viral disease countermeasure for the cultivation fish, diagnostic agents and prevention vaccine are being investigated, but the development has not yet advanced so much. For example, vaccine for the iridovirus which was very often reported in cultivating red sea bream and so on was developed in 1999. However, since the vaccine must be injected into each fish, it is not widely used in farms.
On the other hand, sea lettuce is a kind of green alga growing in seawater, and it occurs in mass by the eutrophication which contains much nitrogen and phosphorus in beach and a seawater lake. Therefore, the existence of sea lettuce has been regarded as one of indications to show the eutrophication state of a sea area conventionally. It is assumed that self-purification action by sea lettuce is very important since sea lettuce takes in nitrogen and phosphorus for proliferation. On the other hand, the present ituation is that people rack their brains about disposal of sea lettuce, and its effective utilization is desired. From this point of view, sea lettuce is a practically safe food source which is partly utilized as food, and the basic study aiming at application to feed additives for domestic animal is started, paying attention on nutrition components such as proteins, vitamins, minerals, and so on. However, decisive effective utilization of sea lettuce has not been found yet, and development of the effective usage that is aiming at mass consumption is expected.
The present inventors have investigated in the past for making good use of sea lettuce. As a result, the present inventors have found that D-cysteinolic acid, which is a sulfur-containing amino acid in sea lettuce, reduces internal neutral fat and inhibits the Fentton reaction which generates harmful radicals, and filed already patent applications based on the observations. (Jpn. Pat. Appln. Publication Nos. 2000-202404 and 2000-103724).
However, in terms of effective utilization of sea lettuce, more sufficient analysis and assessment are expected at various points of view about components of sea lettuce.
In view of the circumstances mentioned above, the present invention has been achieved as part of a study to search bioactive components of sea lettuce and to promote effective use thereof.